Steam pyrolysis is a widely practiced process for cracking petroleum oil, however, it is energy intensive, not very selective, produces coke and releases significant amounts of carbon dioxide into the air. Thus, chemical manufacturers have long recognized a need for an alternative hydrocarbon cracking process. One alternative to the steam pyrolysis process is catalytic cracking.
Government regulations have been and are currently being introduced throughout the world to reduce the sulfur content in gasoline. In most refineries, the fluid catalytic cracking (FCC) process is the major contributor of sulfur to the refinery gasoline pool. While there is a significant amount of research currently being done to improve desulfurization catalysts, there also exists a need to improve refinery processes to achieve a desulfurized gasoline at greater yields and improved efficiencies and costs.
Typical FCC processes provide multiple product streams, including fuel gas, gasoline, light cycle oil, heavy cycle oil and coke. While a major portion of the sulfur that is originally present in the feedstock exits the FCC process as hydrogen sulfide in the fuel gas fraction, the gasoline fraction can also include significant amounts of sulfur.
In a typical catalytic cracking unit, petroleum-derived hydrocarbons are catalytically cracked in the presence of a catalyst to obtain gasoline as the main product, a small amount of LPG, and cracked gas oil. Coke is deposited on the catalyst and is then burnt away with air to regenerate the catalyst, thereby allowing the catalyst to be recycled to the reaction zone for reuse in the process. In a typical refinery, between 30% and 50% of the refinery gasoline production comes from an FCC unit. This stream is typically also responsible for up to about 90% of the sulfur present in gasoline.
In a typical FCC process, light higher value products can typically be selectively increased by increasing the reaction zone temperature, which in turn causes an increase in the contribution of thermal cracking and leads to increased production of lighter products. Known FCC methods, however, typically do not selectively produce sufficient amounts higher value light-fraction products. For example, the high-temperature cracking reaction of the FCC process typically will also result in the thermal cracking of petroleum oils, thereby increasing the yield of dry gases from feedstock oils. For example, in one specific type of FCC process, referred to as a Deep Catalytic Cracking (“DCC”), higher temperatures and increased amounts of steam are used. Thermal cracking in the DCC process, however, is less selective and produces large amounts of relatively low value products, such as hydrogen, methane, ethane, and ethylene, in the “wet gas” (which also contains the H2 and C1-C4 products). Wet gas compression often limits subsequent refinery operation.
MOM Thiophene, benzothiophene, and alkyl derivatives thereof, are among the most abundant organosulfur compounds found in gasoline, generally accounting for more than 80% of the total sulfur that is present in the gasoline fraction. It is these sulfur containing compounds, rather than the sulfides, disulfides and mercaptans, that typically remain after desulfurization processes. The presence of organosulfur compounds in gasoline products typically result in toxic emissions and inefficient performance of exhaust catalysts.
Attempts to reduce sulfur in gasoline produced from an FCC unit have various drawbacks. For example, reducing the end boiling point of the gasoline stream (known as “undercutting”) can reduce the overall sulfur content of the fraction, however this also results in a reduction of gasoline yield, loss of motor octane number (MON), and a reduction in the cetane number of the light cycle oil (LCO). Similarly, hydrotreatment of the FCC petroleum feedstock is an option for removal of sulfur, however this treatment typically involves substantial capital expenditures and typically results in olefin saturation and reduction of the MON.
Additional methods have been designed to produce low sulfur gasoline and some drawbacks associated with them include the following. Selective adsorption of sulfur containing compounds can be carried out at low temperatures, in an attempt to avoid the saturation of alkenes and arenes, which typically prevail during hydrodesulfurization catalysis. However, available processes based on existing catalyst materials have shown limited adsorption capacities and selectivities for thiophenes, which represents a particular challenge with respect to desulfurization.
The reaction of feed oil and a catalyst over a short contact time causes a decrease in the conversion of higher value light-fraction products to light-fraction paraffins, believed to be due to the inhibition of hydrogen transfer reactions. During reactions having a short contact time, the conversion of petroleum oils to light-fraction oils is typically not greatly increased. Furthermore, the use of pentasil-type zeolites can enhance the yield of light-fraction hydrocarbons by excessive cracking of the gasoline once it is produced. Therefore, it is difficult to produce a high yield of the higher value light-fraction products from heavy fraction oils by using either of these known techniques. Thus, there is a need to develop new methods to optimize production conditions where the reaction time is optimized with a view to produce certain desired end products.
In general, a major difficulty with the FCC process is that, in designing the process, the operator seeks to maximize the reactor and stripper temperatures, while at the same time seeking to minimize the regenerator temperature. Controlling the temperatures of both of the devices during this process in this manner does not effectively occur in conventionally heat balanced operations because any increase in the reactor temperature essentially leads to a subsequent increase in the regenerator temperature, thereby resulting in a heat imbalance. Therefore, a need exists for appropriate control systems that allow appropriate heat-balances in a FCC unit.
Additionally, in typical FCC processes, control of the FCC catalyst and FCC additive usage is typically manually augmented during the refining process to control the emissions and product mix. In other words, there are currently no systematic feedback mechanisms for optimizing FCC processes, thus requiring for manual control of the process by an operator. Thus there exists a need for methods to monitor and optimize FCC catalyst and FCC additive usage during processing.
Due to the uncertainties of the chemical composition of feedstocks that can be supplied to the FCC unit, both the emissions from the process and the product mixture may vary from the desired target emissions and product mixture over the course of the refining process. As a result, system operators are typically required to closely monitor the system outputs and be constantly available to make manual adjustments to the FCC catalyst and FCC additive injection schedules and other operating parameters, as needed. Operating the FCC unit in this manner, where the operator is required to constantly manually monitor the process and make adjustments thereto, poses a significant challenge when the system is being operated under severe conditions. Thus, a need exists to provide a method for remotely monitoring and controlling the overall FCC process and allowing process models to predict necessary adjustments to the catalyst injection schedule based upon system outputs, while at the same time reducing the reliance on human interactions, such as monitoring and making manual changes to the catalyst injection schedule.
Moreover, current FCC units and processes do not necessarily model and optimize process variables for maximizing conversion of feedstocks to low-sulfur gasoline and minimizing FCC catalyst and FCC additive usage, particularly when the FCC unit is operated at a severe mode. Therefore, a need exists for an automated process for the optimization of the unit and adjustment of various process variables, such as feed rate, feed quality, set of processing objectives and FCC catalyst and FCC additive usage and selection based upon other unit constraints (e.g., wet gas compressor capacity, fractionation capacity, air blower capacity, reactor temperature, regenerator temperature, catalyst circulation).